Transmission associated with electric power generation and consumption typically requires a series of power conversion steps which add to the cost and reduce the efficiencies of operations. The conventional transmission path typically includes a high voltage alternating current (AC) segment, typically on the order of 14 KV, followed by one or more voltage reductions or step downs. Depending on the specific application, there may be multiple voltage conversions and multiple conversions between direct current (DC) and AC. Some power delivery systems rely on AC transmission over relatively short distances to reduce losses. Those segments may carry power at 300 to 400 Volts or higher. Frequently the service is delivered to residential and business environments via a further step down of AC. Subsequently, components of individual appliances perform further voltage conversions to meet requirements. For example, many electronic appliances include power supplies or charging units that convert power to DC at a specified voltage. While it is widely recognized that individual conversions involve inefficiencies, these are typically tolerated because they are on an individual basis, the associated costs are small and the costs are not separately identified to the consumer.
On the other hand, when the service is delivered to commercial/industrial complexes that utilize large amounts of power in similar DC applications, the inefficiencies of power conversions can be taken into consideration by optimizing the design for power transmission and distribution. Nonetheless, even with economies of scale, there have been limited means for improving the efficiency of power delivery. This is partly due to the number of conversions required to deliver power of the correct form factor to DC inputs. Both the transformers and the AC-DC semiconductor converters used in these systems have losses which can only be avoided by eliminating the voltage transformations. Yet it has been necessary to employ multiple conversions in order to optimize overall efficiencies. This is particularly true for segments which transmit power from high voltage distribution points within a commercial complex to locations of power consumption. An additional transformation is often required to implement energy storage devices into these systems. In many cases batteries are used for energy storage which requires a relatively low voltage DC feed. The incoming AC is converted to a low voltage DC power. The batteries are connected at the DC side and then the power is converted back to AC at a higher voltage for low loss power transmission.
Data centers are exemplary of power applications for which the inventive concepts are suited. The term data center refers to the wide variety of complexes which house servers to make readily available information systems and/or large volumes of data for access. The information may be commercial in nature, e.g., credit card data, travel reservation systems, or personal, including email and pictures stored on a server. Small data centers may be housed within conventional office buildings. Larger data centers may be in specifically designed buildings.
In some contexts such data centers are referred to as server farms or Internet Data Centers. In addition to containing large amounts of electronic equipment, data centers include a variety of back-up equipment and cooling equipment for controlling temperature and humidity of the server environment.
As the size of individual data centers grows, as measured by the number of servers in use, power requirements will continue to increase. A typical data center may occupy ten to one hundred thousand square feet and consume megawatts of power. It has been estimated that data centers in the United States presently consume approximately three percent of all electric power generated in the United States. The global market for only the power and cooling components used in data centers worldwide may already exceed forty billion U.S. Dollars annually. Continued growth is expected.
Design of power distribution systems within data centers is made more complex than other industrial power applications because of the demand for high levels of reliability. For example, distribution systems within a data center complex will commonly incorporate alternate power sources, as well as devices which store energy to provide power for brief periods when there are needs to transition from a disrupted supply of power to a secondary source of power.
An exemplary prior art power distribution system which primarily utilizes AC transmission is shown in FIG. 1A. A trunk line brings in high voltage (e.g., 14 kV) power to a transformer 2 which steps the voltage down to within the range of 200 to 500 volts. Energy storage, located at a central point before distribution branches, typically requires a conversion 4 of the AC power to DC power. The subsequent energy storage 6 may be in the form of an Uninterruptible Power Supply (UPS). Batteries in the UPS are charged with incoming power after an initial AC to DC conversion. The DC output from the UPS then undergoes a conversion 8 to a high voltage AC supply for transmission 10 within the data center, e.g., on the order of 50 m, in order to minimize losses in subsequent segments of the distribution system as the power is delivered to clusters of servers. When the service is delivered to a point local to a cluster of server racks, the power undergoes still another conversion 12 to provide the relatively low voltage, high current source DC input needed to power the servers 14 in the cluster. A typical cluster of forty server racks may draw over two megawatts of power.
The power distribution system of FIG. 1A experiences significant losses resulting from at least three conversions between AC and DC power and reductions or step-ups to levels which are suitable for transmission or input to an array of different low voltage devices. Each conversion generates heat which must be removed from the data center in order to control the environment for reliable equipment operation. The cost of power transmission inefficiencies are compounded by power demands necessary for cooling and humidity control.
Another exemplary power distribution system, shown in FIG. 1B, has an initial segment like that of the system shown in FIG. 1A, converting the high voltage (e.g., 14 KV) power received from the trunk line with a transformer 20 which steps the voltage down to within the range of 200 to 500 volts. This is followed by a conversion 4 of the AC power to DC power. Energy storage 6, such as a UPS is located at a central point before power distribution branches. Instead of converting the DC output from the UPS, the DC power undergoes DC transmission 16 directly to a point in the proximity of clusters of servers 14. At a point local to each cluster of servers 14 there is a DC to DC voltage step down 18 to meet input requirements of the equipment. Such an AC DC distribution arrangement still requires multiple conversions, but can be more energy efficient than the system of FIG. 1A.
Losses can be further reduced when the power distribution system incorporates a UPS that outputs a DC voltage consistent with the input voltage needed at the servers, e.g., 12 volts.
Such an arrangement, as illustrated in FIG. 1C, has an initial segment like that of the system shown in FIGS. 1A and 1B, having a transformer 2 convert high voltage (e.g., 14 KV) power received from a trunk line down to the range of 200 to 500 volts. This is followed by a conversion 4 of the AC power to DC power. Energy storage 6, provided with a UPS, is located at a central point before power distribution branches. The UPS provides as an output a DC voltage 24 suitable for input to the servers 14 in a cluster prior to transmission 26 to the servers 14. This arrangement requires that the segment of transmission path over which the low voltage DC power is carried be relatively short in order to avoid significant losses.